1. Technical Field
The present invention relates to a method of manufacturing an organic electroluminescent device (xe2x80x9corganic EL devicexe2x80x9d) and to an ink composition for an organic EL device.
2. Background Technology
The basic structure of an organic EL device includes a cathode and an anode sandwiching both sides of an organic thin film containing a fluorescent organic compound. When a voltage is applied between the cathode and anode, electrons from the cathode and holes from the anode are injected into the organic thin film. Driven by the applied electric field, the charge carriers move through the organic thin film, where they recombine. Excitons are generated by the energy given during recombination. When these excitons return to their ground state, energy is emitted in the form of a fluorescence or phosphorescence. This phenomenon is known as electroluminescent (EL) emission. Devices that use such EL emission are organic EL devices.
An organic molecule is used for the organic thin film. The use of an organic molecule makes it easy to control the color of emitted light. Materials can be used in myriad combinations to optimize the structure of the organic thin film for use in an organic EL device. For this reason, expectations are high in the display market for the realization of organic EL devices as low-voltage driven, high luminous efficiency, self-emitting elements.
The organic molecules that form the organic thin film can be broadly divided into two classes: the small molecule class or the polymer class. Organic thin films comprised of an organic small molecule are generally formed by vacuum deposition techniques, whereas polymeric organic thin films are generally formed by spin-coating or ink-jet printing a polymer that has been dissolved or dispersed in a solvent.
Incidentally, regardless of the type of organic molecule used, the performance of an organic EL device is significantly affected by the smoothness of the organic thin film that is formed and by the adhesion between organic thin films that are disposed in layers. These factors, which affect the uniformity of emissions and the state of contact with electrodes or other organic thin films, are strongly correlated to organic EL device degradation. For this reason, the technology for forming organic thin films is an important issue for improving the characteristics and lifetime of organic EL devices, and, hence, is the subject of considerable study.
Japanese Laid Open to the Public No. Hei 5-182764 describes a method of manufacturing an organic electroluminescent element wherein a light-emitting layer, formed by vacuum-depositing an organic compound, is heat-treated at a temperature of 50xc2x0 C. or more but not more than the melting point of the organic compound, so as to form a microcrystalline condensed structure. Hence, the deterioration of organic EL device lifetime and the changes in organic EL device characteristics caused by heat generated during device driving are suppressed thereby.
Moreover, Japanese Laid Open to the Public No. Hei 11-40352 describes a method of manufacturing an organic EL device, where the organic layer, formed by vacuum deposition and including a low molecule dye, is heat-treated at a temperature range of xe2x88x9220 to +20xc2x0 C. from the glass transition temperature of the material in the organic layer having the lowest glass transition temperature. Hence, the lifetime of the organic EL device can be extended thereby.
Furthermore, Japanese Laid Open to the Public No. Hei 11-40358 describes a method of manufacturing an organic EL device where an ink composition for the organic EL device, the ink composition including a precursor of a conjugated polymeric organic compound, is ejected by an ink-jet method. The ejected ink composition subsequently is polymerized by heat treatment so as to form the light-emitting layer.
However, it is difficult to control the film quality of organic thin films created using an ink-jet method, and the surface smoothness of thin films at interfaces and the mutual adhesion of organic thin films disposed atop one another in layers was particularly poor. Since both smoothness and adhesion affect the characteristics of light-emitting elements, these problems made it difficult to extend the lifetime of organic EL devices. In addition, heat generated during driving an element brings changes in the film structure. These structural changes have an adverse impact on organic EL devices, as they degrade the elements, reduce emission efficiency, change emission colors, and so forth.
The present invention, which takes the aforementioned situation into account, is intended to provide a method of manufacturing an organic EL device and an ink composition for an organic EL device having a long lifetime, as well as stable emission luminance and other superior emissive properties. The present invention proposes to achieve this by improving the smoothness of the organic thin film comprising the light-emitting layer and by improving the adhesion of organic thin films to one another where such films are disposed in layers.
Therefore, according to a preferred embodiment of a method of manufacturing an organic EL device of the present invention, a method of manufacturing an organic EL device having a light-emitting layer formed between a first electrode and a second electrode comprises forming a light-emitting layer on a first electrode by ejecting and heat-treating an ink composition for the organic EL device, including an organic light-emitting material and at least one type of a high boiling point solvent having a boiling point of 200xc2x0 C. or higher.
After the ink composition for the organic EL device is thus ejected upon the first electrode, it is heat-treated, thereby smoothing the surface of the organic film, which is made up of droplets of the ink composition for the organic EL device, and enabling the manufacture of an organic EL device having excellent emissive properties and long lifetime.
Moreover, by pre-heat treating the organic thin film before a second electrode is formed, it is possible to approximate the film structure of an organic EL device in the driving state. Hence, distortion and other changes in film structure due to heat generated during driving an organic EL device can be minimized by this pre-heat treatment. Thus, these changes do not adversely affect element characteristics.
Furthermore, the aforementioned organic EL device is preferably manufactured by ejecting the ink composition for the organic EL device with an ink-jet apparatus.
Furthermore, the method of manufacturing the aforementioned organic EL device includes forming a plurality of pixels, including a plurality of dots, upon a substrate having a first electrode, each dot being separated by a barrier (xe2x80x9cbankxe2x80x9d).
Furthermore, the aforementioned method of manufacturing the organic EL device comprises a method of controlling the quality of the organic thin film by performing heat treatment after the ink composition for the organic EL device is ejected and while a high boiling point solvent is still present. Heat treatment is preferably performed at a temperature at or below the boiling point of the high boiling point solvent. More preferable, heat treatment is performed at xe2x88x9215 to +40xc2x0 C. from the glass transition temperature of the organic light-emitting material and, moreover, at not more than the boiling point of the high boiling point solvent.
By thus performing heat treatment with a high boiling point solvent remaining within the organic thin film formed from droplets of the ink composition for the organic EL device, the surface of the organic thin film can be smoothed, yielding an organic thin film having superior smoothness.
Moreover, the ink composition for an organic EL device in a preferred embodiment of the present invention contains an organic light-emitting material and a high boiling point solvent, the high boiling point solvent having a boiling point in the range from 200xc2x0 C. to 400xc2x0 C. at atmospheric pressure. In addition, the viscosity of the ink composition for the organic EL device is preferably 1 to 20 mPa.s. An ink composition for the organic EL device having a viscosity of 1 to 20 mPa.s can be smoothly ejected from a nozzle hole of an ink-jet apparatus.